Triacylglycerols (TAGs) produced by plants are one of the most energy-rich and abundant forms of reduced carbon (carbon bonds that store energy) available from nature.
Conventional oils from crop plant species are a variety of lipids and fatty acids including triacylglycerols (TAGs) with 3 long acyl chains. TAGs show structural similarity and similar energy content compared to aliphatic acyclic components of diesel fuel molecules. Given their chemical structure similarities, plant oils represent a logical substitute as a renewable energy source for conventional petroleum (crude oil)-derived diesel, a non-renewable energy source, Durrett, Benning, Ohlrogge, Plant Journal. 2008, 54(4):593-607, herein incorporated by reference. Time estimates vary widely on when global peak production of nonrenewable conventional petroleum begins declining, some estimate reductions in production as early as 2025. Despite a range of estimates, there is universal agreement that there is a finite limit of crude petroleum and natural gas in the earth's crust available for profitable extraction.
Currently, plant oils are converted to fatty acid esters that are used as biofuel. The resulting fuel is commonly referred to as biodiesel, and offers many advantages over conventional diesel. Chief among these is that biodiesel is derived from renewable sources. In addition, the production and subsequent consumption of biodiesel results in arguably less greenhouse gas emission compared to conventional diesel. However, the widespread adoption of biodiesel faces a number of challenges. One major challenge is the limited supply of biodiesel feedstocks, i.e. plant TAGs. Thus, plant oil production in general needs to be greatly increased for biodiesel to replace a major proportion of the current and future fuel needs of the world.
Another major challenge is providing plant feedstocks that do not require the conversion of plant oils into fatty acid esters, i.e, transesterification (see, FIG. 1). This conversion is necessary because plant oils produced in sufficient quantity for commercial use, i.e. current biodiesel feedstocks including from feedstocks from oil seed crop plants, such as soybean seeds, rapeseeds, and sunflower seeds, are unsuitable for direct use as fuel in unmodified diesel engines. In particular, these plant oils are too viscous for use in modern diesel engines. High viscosity of plant TAGs results in a number of problems including poor atomization in engines, leading to incomplete combustion and subsequent problems, such as carbon deposition and coking. Further, during the high temperatures of combustion the acyl chains in TAGs polymerize leading to gum formation.
Transesterification of plant TAGs into useable fuel also results in a number of problems associated with efficient biofuel production. In particular, transesterification results in yield losses, such as losses due to triglyceride saponification and methyl ester dissolution in glycerol, as well as uncertainty over the glycerol credit itself to cover unit operation costs. Further, mixtures of unconverted monoglyceride, diglyceride and triglyceride impurities affect engine performance. Residual alcohol content in biodiesel attacks natural rubber seals and gaskets, i.e. unmodified for use with currently available biodiesel. Biodiesel also undergoes chemical and biological modification over time that affects its quality during long-term storage.
Thus, there is a need for an increase in production of biodiesel feedstock and a need of higher quality biofuel feedstock before biodiesel can replace a major proportion of current and future biodiesel needs globally and in the United States.